This proposal aims at establishing a working model of the performance of the sino-aortic reflex system: (1) The experimental study begins with filling in the information gaps such as the reflex controls of cardiac pressure-volume relationship and systemic and pulmonary vascular capacities. Synthesis of the results with earlier information produces an initial crude model under a set of simplifying assumptions. (2) The study then moves to quantification of the parametric controls of all the major components of circulation in intact dogs. Initially, just the sinus and/or aortic receptors are perturbed in conscious dogs by head level change, neck suction and aortic constriction with implanted cuffs. These data are compared with the crude model's responses to the same perturbances. The difference revises some of the hypotheses, model parameter values, or both. In turn, the model now adjusted to the real system permits estimation of reflex changes in some of the plant parameters immeasurable in intact dogs. The study repeated after anesthetizing the same dogs should define the modifications by anesthesia. (3) Finally, the same dogs are hemorrhaged, first in conscious and then anesthetized conditions. The data are compared with the revised model's output, revealing the reflex performances peculiar to hemorrhage. Whether the variation among subjects derives from the reflex system or the cardiovascular plant is also analyzed in this project.